Signal transduction is the general process by which cells repond to extracellular signals (hormones, neurotransmitters, growth and differentiation factors, etc.) through a cascade of biochemical reactions. The process begins with the binding of the signal molecule to a cell membrane receptor and ends with the activation of an intracellular target molecule. Such processes regulate all types of cell functions including cell proliferation, differentiation, and gene transcription, as well as oncogenic transformation.
Many growth factor receptors, including epidermal growth factor, platelet-derived growth factor, and fibroblast growth factor contain intrinsic protein kinase activities. When the polypeptide growth factor binds to the receptor, it triggers the autophosphorylation of a tyrosine residue on the receptor. It is believed that these phosphorylated sites are recognition sites for the binding of other cytoplasmic signaling proteins in the signaling pathway that eventally links the initial receptor activation at the cell surface to the activation of a specific intracellular target molecule. These signaling proteins contain a common domain refered to as a src homology 2 (SH2) domain. SH2 domains are found in a variety of signaling molecules and oncogenic proteins such as phospholipase C-.gamma., Ras GTP-ase activating protein, and pp60.sup.c-src (Lowenstein, E. J. et al. (1992) Cell 70:431-42).
A subfamily of these receptor binding, cell signaling proteins has been found in humans and is called Grb2 (growth factor receptor binding protein). Grb2 homologs have been identified in rat (ASH), C. elegans (Sem-5), and Drosophila (Drk) (Lowenstein et al., supra; Matuoka, K. et al. (1992) Proc, Natl. Acad. Sci. 89:9015-19). Grb2 and its homologs contain an SH2 domain flanked by SH3 (src-homology 3) domains. The homology between these domains from species to species ranges between 45 and 60% indicating an evolutionarily well conserved family of proteins.
Grb2 and its homologs appear to function in the Ras signaling pathway by mediating an interaction between the phosphorylated EGF receptor and a Ras activating protein that converts GDP to bound GTP (Egan, S. E. et al. (1993) Nature 363:45-48; Laszlo, B. and Downward, J. (1993) Cell 73:611-20). Activated Ras stimulates normal growth and differentiation processes in some cell types and oncogenic transformation and the development of cancer in others. Binding studies conducted with isolated domains from Grb proteins have determined that binding of the SH2 domain of Grb to the phosphotyrosine region of the EGF receptor and the binding of the two flanking SH3 domains to the Ras activating protein is simultaneous. Specific amino acid substitutions in the SH2 and SH3 domains of Grb proteins have further defined the important functional amino acid residues (Matuoka, et al., supra; Egan, et al., supra).
The discovery of a novel growth factor receptor binding protein provides a means to investigate cell growth and differentiation processes under normal and disease conditions. Such molecules related to a growth factor receptor binding protein satisfy a need in the art by providing new diagnostic or therapeutic compositions useful in cancer, immunological disorders and disorders of the sympathetic nervous system.